Surfactants constitute a class of amphiphilic molecules whose physical and chemical properties provide the opportunity to manipulate conditions for the controlled emulsion polymerization of a variety of functional monomers. The resultant soluble macromolecules reflect, in part, physical and chemical properties derived % from the surfactant-mediated polymerization conditions, including molecular weight and polydispersity. Surfactants also allow for the sequestration and separation of valuable compounds from incompatible or dissimilar media, e.g., detergent and waste recovery applications, wetting agents, thickeners, tertiary petrochemical recovery and processing.
The ability of surfactants to arrange into ordered, low energy spherical micelles, planar lamellae or higher ordered structures, e.g., stacked lamellae, tubes/rods, can be used to provide a templating surface from which ordered meso- and microstructural materials with tailored properties can be generated. The physical arrangement of the final material, such as zeolites, nanoparticles, and biomaterials, reflects the ordering of the template structure. The proper selection of conditions, e.g., solvent media and temperature, allows for the control not only of the growth of such structures but also their properties such as electronic activity, porosity and permeability.
While ordered surfactants can provide a desirable template structure, realizing the desired properties of the final material requires that the surfactant template be removed. The process of removal can present a significant technical challenge since the surfactant materials are often large and relatively non-volatile amphiphiles that are difficult to remove by means other than harsh thermal or corrosive chemical treatments with attendant damage to the final material.
Some examples of surfactants exist in the literature which may be degraded under mild chemical conditions. Generally, a change in the pH of a system has been used to degrade surfactants with acid-sensitive functionalities, such as dioxolanes (see (a). Ono, D.; Masuyama, A.; Okahara, M. J. Org. Chem. 1990,-55, 4461; (b) Jaeger, D. A.; Li, B.; Clark, Jr., T. C. Langmuir 1996, 12, 4314; (c) Jaeger, D. A.; Wettstein, J.; Zafar, A. Langmuir 1998, 14, 1940; (d) Piasecki, A.; Sokolowski, A.; Burczyk, B.; Gancarz, R.; Kotlewska, U. Langmuir 1997, 13, 143). Acid cleavable surfactants have been successfully utilized for removal of detergents used in preparation of hydrophobic protein samples for MALDI-MS analysis (see (a) Caprioli, R. M.; Porter, N. A.; Norris, J. L. WIPO Patent 2002, WO 02/097393 A2. (b) Norris, J. L.; Porter, N. A.; Caprioli, R. M. Anal. Chem. 2003, 75, 6642), and a recent report details the use of condensable surfactants with masked functionalities, developed for the production of templated organo-silicates with precise interior surface functionalities which are exposed by partial surfactant degradation via hydrolysis after vitrification and calcinations (Zhang, Q.; Ariga, K.; Okabe, A.; Aida, T. J. Am. Chem. Soc. 2004, 126, 988).
Base-sensitive surfactants include functional groups such as esters or 13-ammonium ketones (see (a) Jaeger, D. A.; Finley, T.; Walter, M. R.; Martin, C. A. J. Org. Chem. 1986, 51., 3956. (b) Lindstedt, M.; Allenmark, S.; Thompson, R. A.; Edebo, L. Antimicrob. Agents Chemother. 1990, 34, 1949). Efforts toward surfactants which allow for reversible control of surface activity and interfacial surface tension through incorporation of redox-active ferrocenyl moieties have also been reported (see (a) Gallardo, B. S.; Hwa, M. J.; Abbott, N. L. Langmuir 1995, 11, 4209; (b) Gallardo, B. S.; Metcalfe, K. L.; Abbott, N. L. Langmuir 1996, 12, 4116; (c) Bennett, D E.; Gallardo, B. S.; Abbott, N. L. J. Am. Chem. Soc. 1996, 118, 6499; (d) Gallardo, B. S.; Abbott, N. L. Langmuir 1997, 13, 203).
Surfactants sensitive to non-chemical stimuli include photosensitive azosulfonates (Mezger, T.; Nuyken, O.; Meindl, K.; Wokaun, A. Prog. Org. Coat. 1996, 29, 147) and thermally sensitive amine oxides (Hayashi, Y.; Shirai, F.; Shimzu, T.; Nagano, Y.; Teramura, K. J. Am. Oil Chemists Soc. 1985, 62, 555).
The examples above require conditions, either chemical or physical, that may be adverse for removing the surfactant template without damage to any hierarchical material structure and are not applicable under conditions requiring neutral pH, or those conditions where oxidative or reductive environments are unsuitable for material processing.